Current waveforms for anti-bradycardia pacing for a subcutaneous implantable cardioverter-defibrillator

ABSTRACT

A power supply for an implantable cardioverter-defibrillator for subcutaneous positioning between the third rib and the twelfth rib and using a lead system that does not directly contact a patient&#39;s heart or reside in the intrathoracic blood vessels and for providing anti-bradycardia pacing energy to the heart, comprising a capacitor subsystem for storing the anti-bradycardia pacing energy for delivery to the patient&#39;s heart; and a battery subsystem electrically coupled to the capacitor subsystem for providing the anti-bradycardia pacing energy to the capacitor subsystem.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/011,506, filed Nov. 5, 2001, abandoned; which is acontinuation-in-part of U.S. application Ser. No. 09/663,607, filed Sep.18, 2000, now U.S. Pat. No. 6,721,597; and U.S. application Ser. No.09/663,606, filed Sep. 18, 2000, now U.S. Pat. No. 6,647,292, thedisclosures of which are all incorporated herein by reference.

In addition, this application is related to U.S. application Ser. No.10/011,860, filed Nov. 5, 2001, now U.S. Pat. No. 7,092,754; U.S.application Ser. No. 10/011,958, filed Nov. 5, 2001, abandoned; and U.S.application Ser. No. 10/015,202, filed Nov. 5, 2001, now U.S. Pat. No.6,952,610; the disclosures of which applications are hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for performingelectrical cardioversion/defibrillation and optional pacing of the heartvia a non-transvenous system.

BACKGROUND OF THE INVENTION

Defibrillation/cardioversion is a technique employed to counterarrhythmic heart conditions including some tachycardias in the atriaand/or ventricles. Typically, electrodes are employed to stimulate theheart with electrical impulses or shocks, of a magnitude substantiallygreater than pulses used in cardiac pacing. Shocks used fordefibrillation therapy can comprise a biphasic truncated exponentialwaveform. As for pacing, a constant current density is desired to reduceor eliminate variability due to the electrode/tissue interface.

Defibrillation/cardioversion systems include body implantable electrodesthat are connected to a hermetically sealed container housing theelectronics, battery supply and capacitors. The entire system isreferred to as implantable cardioverter/defibrillators (ICDs). Theelectrodes used in ICDs can be in the form of patches applied directlyto epicardial tissue, or, more commonly, are on the distal regions ofsmall cylindrical insulated catheters that typically enter thesubclavian venous system, pass through the superior vena cava and, intoone or more endocardial areas of the heart. Such electrode systems arecalled intravascular or transvenous electrodes. U.S. Pat. Nos.4,603,705; 4,693,253; 4,944,300; and 5,105,810, the disclosures of whichare all incorporated herein by reference, disclose intravascular ortransvenous electrodes, employed either alone, in combination with otherintravascular or transvenous electrodes, or in combination with anepicardial patch or subcutaneous electrodes. Compliant epicardialdefibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and5,618,287, the disclosures of which are incorporated herein byreference. A sensing epicardial electrode configuration is disclosed inU.S. Pat. No. 5,476,503, the disclosure of which is incorporated hereinby reference.

In addition to epicardial and transvenous electrodes, subcutaneouselectrode systems have also been developed. For example, U.S. Pat. Nos.5,342,407 and 5,603,732, the disclosures of which are incorporatedherein by reference, teach the use of a pulse monitor/generatorsurgically implanted into the abdomen and subcutaneous electrodesimplanted in the thorax. This system is far more complicated to use thancurrent ICD systems using transvenous lead systems together with anactive can electrode and therefore it has no practical use. It has infact never been used because of the surgical difficulty of applying sucha device (3 incisions), the impractical abdominal location of thegenerator and the electrically poor sensing and defibrillation aspectsof such a system.

Recent efforts to improve the efficiency of ICDs have led manufacturersto produce ICDs which are small enough to be implanted in the pectoralregion. In addition, advances in circuit design have enabled the housingof the ICD to form a subcutaneous electrode. Some examples of ICDs inwhich the housing of the ICD serves as an optional additional electrodeare described in U.S. Pat. Nos. 5,133,353; 5,261,400; 5,620,477; and5,658,321, the disclosures of which are incorporated herein byreference.

ICDs are now an established therapy for the management of lifethreatening cardiac rhythm disorders, primarily ventricular fibrillation(V-Fib). ICDs are very effective at treating V-Fib, but are therapiesthat still require significant surgery.

As ICD therapy becomes more prophylactic in nature and used inprogressively less ill individuals, especially children at risk ofcardiac arrest, the requirement of ICD therapy to use intravenouscatheters and transvenous leads is an impediment to very long termmanagement as most individuals will begin to develop complicationsrelated to lead system malfunction sometime in the 5-10 year time frame,often earlier. In addition, chronic transvenous lead systems, theirreimplantation and removals, can damage major cardiovascular venoussystems and the tricuspid valve, as well as result in life threateningperforations of the great vessels and heart. Consequently, use oftransvenous lead systems, despite their many advantages, are not withouttheir chronic patient management limitations in those with lifeexpectancies of >5 years. The problem of lead complications is evengreater in children where body growth can substantially altertransvenous lead function and lead to additional cardiovascular problemsand revisions. Moreover, transvenous ICD systems also increase cost andrequire specialized interventional rooms and equipment as well asspecial skill for insertion. These systems are typically implanted bycardiac electrophysiologists who have had a great deal of extratraining.

In addition to the background related to ICD therapy, the presentinvention requires a brief understanding of a related therapy, theautomatic external defibrillator (AED). AEDs employ the use of cutaneouspatch electrodes, rather than implantable lead systems, to effectdefibrillation under the direction of a bystander user who treats thepatient suffering from V-Fib with a portable device containing thenecessary electronics and power supply that allows defibrillation. AEDscan be nearly as effective as an ICD for defibrillation if applied tothe victim of ventricular fibrillation promptly, i.e., within 2 to 3minutes of the onset of the ventricular fibrillation.

AED therapy has great appeal as a tool for diminishing the risk of deathin public venues such as in air flight. However, an AED must be used byanother individual, not the person suffering from the potential fatalrhythm. It is more of a public health tool than a patient-specific toollike an ICD. Because >75% of cardiac arrests occur in the home, and overhalf occur in the bedroom, patients at risk of cardiac arrest are oftenalone or asleep and can not be helped in time with an AED. Moreover, itssuccess depends to a reasonable degree on an acceptable level of skilland calm by the bystander user.

What is needed therefore, especially for children and for prophylacticlong term use for those at risk of cardiac arrest, is a combination ofthe two forms of therapy which would provide prompt and near-certaindefibrillation, like an ICD, but without the long-term adverse sequelaeof a transvenous lead system while simultaneously using most of thesimpler and lower cost technology of an AED. What is also needed is acardioverter/defibrillator that is of simple design and can becomfortably implanted in a patient for many years.

SUMMARY OF THE INVENTION

A power supply for an implantable cardioverter-defibrillator forsubcutaneous positioning between the third rib and the twelfth rib andusing a lead system that does not directly contact a patient's heart orreside in the intrathoracic blood vessels and for providinganti-bradycardia pacing energy to the heart, comprising a capacitorsubsystem for storing the anti-bradycardia pacing energy for delivery tothe patient's heart; and a battery subsystem electrically coupled to thecapacitor subsystem for providing the anti-bradycardia pacing energy tothe capacitor subsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is now made tothe drawings where like numerals represent similar objects throughoutthe figures where:

FIG. 1 is a schematic view of a Subcutaneous ICD (S-ICD) of the presentinvention;

FIG. 2 is a schematic view of an alternate embodiment of a subcutaneouselectrode of the present invention;

FIG. 3 is a schematic view of an alternate embodiment of a subcutaneouselectrode of the present invention;

FIG. 4 is a schematic view of the S-ICD and lead of FIG. 1subcutaneously implanted in the thorax of a patient;

FIG. 5 is a schematic view of the S-ICD and lead of FIG. 2subcutaneously implanted in an alternate location within the thorax of apatient;

FIG. 6 is a schematic view of the S-ICD and lead of FIG. 3subcutaneously implanted in the thorax of a patient;

FIG. 7 is a schematic view of the method of making a subcutaneous pathfrom the preferred incision and housing implantation point to atermination point for locating a subcutaneous electrode of the presentinvention;

FIG. 8 is a schematic view of an introducer set for performing themethod of lead insertion of any of the described embodiments;

FIG. 9 is a schematic view of an alternative S-ICD of the presentinvention illustrating a lead subcutaneously and serpiginously implantedin the thorax of a patient for use particularly in children;

FIG. 10 is a schematic view of an alternate embodiment of an S-ICD ofthe present invention;

FIG. 11 is a schematic view of the S-ICD of FIG. 10 subcutaneouslyimplanted in the thorax of a patient;

FIG. 12 is a schematic view of yet a further embodiment where thecanister of the S-ICD of the present invention is shaped to beparticularly useful in placing subcutaneously adjacent and parallel to arib of a patient;

FIG. 13 is a schematic of a different embodiment where the canister ofthe S-ICD of the present invention is shaped to be particularly usefulin placing subcutaneously adjacent and parallel to a rib of a patient;

FIG. 14 is a schematic view of a Unitary Subcutaneous ICD (US-ICD) ofthe present invention;

FIG. 15 is a schematic view of the US-ICD subcutaneously implanted inthe thorax of a patient;

FIG. 16 is a schematic view of the method of making a subcutaneous pathfrom the preferred incision for implanting the US-ICD;

FIG. 17 is a schematic view of an introducer for performing the methodof US-ICD implantation;

FIG. 18 is an exploded schematic view of an alternate embodiment of thepresent invention with a plug-in portion that contains operationalcircuitry and means for generating cardioversion/defibrillation shockwaves;

FIG. 19 is a graph that shows an example of a biphasic waveform for usein anti-bradycardia pacing in an embodiment of the present invention;and

FIG. 20 is a graph that shows an example of a monophonic waveform foruse in anti-bradycardia pacing in an embodiment of the presentinvention.

DETAILED DESCRIPTION

Turning now to FIG. 1, the S-ICD of the present invention isillustrated. The S-ICD consists of an electrically active canister 11and a subcutaneous electrode 13 attached to the canister. The canisterhas an electrically active surface 15 that is electrically insulatedfrom the electrode connector block 17 and the canister housing 16 viainsulating area 14. The canister can be similar to numerous electricallyactive canisters commercially available in that the canister willcontain a battery supply, capacitor and operational circuitry.Alternatively, the canister can be thin and elongated to conform to theintercostal space. The circuitry will be able to monitor cardiac rhythmsfor tachycardia and fibrillation, and if detected, will initiatecharging the capacitor and then delivering cardioversion/defibrillationenergy through the active surface of the housing and to the subcutaneouselectrode. Examples of such circuitry are described in U.S. Pat. Nos.4,693,253 and 5,105,810, the entire disclosures of which are hereinincorporated by reference. The canister circuitry can providecardioversion/defibrillation energy in different types of waveforms. Inone embodiment, a 100 uF biphasic waveform is used of approximately10-20 ms total duration and with the initial phase containingapproximately ⅔ of the energy, however, any type of waveform can beutilized such as monophasic, biphasic, multiphasic or alternativewaveforms as is known in the art.

In addition to providing cardioversion/defibrillation energy, thecircuitry can also provide transthoracic cardiac pacing energy. Theoptional circuitry will be able to monitor the heart for bradycardiaand/or tachycardia rhythms. Once a bradycardia or tachycardia rhythm isdetected, the circuitry can then deliver appropriate pacing energy atappropriate intervals through the active surface and the subcutaneouselectrode. Pacing stimuli can be biphasic in one embodiment and similarin pulse amplitude to that used for conventional transthoracic pacing.

This same circuitry can also be used to deliver low amplitude shocks onthe T-wave for induction of ventricular fibrillation for testing S-ICDperformance in treating V-Fib as is described in U.S. Pat. No.5,129,392, the entire disclosure of which is hereby incorporated byreference. Also the circuitry can be provided with rapid induction ofventricular fibrillation or ventricular tachycardia using rapidventricular pacing. Another optional way for inducing ventricularfibrillation would be to provide a continuous low voltage, i.e., about 3volts, across the heart during the entire cardiac cycle.

Another optional aspect of the present invention is that the operationalcircuitry can detect the presence of atrial fibrillation as described inOlson, W. et al. “Onset And Stability For Ventricular TachyarrhythmiaDetection in an Implantable Cardioverter and Defibrillator,” Computersin Cardiology (1986) pp. 167-170. Detection can be provided via R-RCycle length instability detection algorithms. Once atrial fibrillationhas been detected, the operational circuitry will then provide QRSsynchronized atrial defibrillation/cardioversion using the same shockenergy and waveshape characteristics used for ventriculardefibrillation/cardioversion.

The sensing circuitry will utilize the electronic signals generated fromthe heart and will primarily detect QRS waves. In one embodiment, thecircuitry will be programmed to detect only ventricular tachycardias orfibrillations. The detection circuitry will utilize in its most directform, a rate detection algorithm that triggers charging of the capacitoronce the ventricular rate exceeds some predetermined level for a fixedperiod of time: for example, if the ventricular rate exceeds 240 bpm onaverage for more than 4 seconds. Once the capacitor is charged, aconfirmatory rhythm check would ensure that the rate persists for atleast another 1 second before discharge. Similarly, terminationalgorithms could be instituted that ensure that a rhythm less than 240bpm persisting for at least 4 seconds before the capacitor charge isdrained to an internal resistor. Detection, confirmation and terminationalgorithms as are described above and in the art can be modulated toincrease sensitivity and specificity by examining QRS beat-to-beatuniformity, QRS signal frequency content, R-R interval stability data,and signal amplitude characteristics all or part of which can be used toincrease or decrease both sensitivity and specificity of S-ICDarrhythmia detection function.

In addition to use of the sense circuitry for detection of V-Fib orV-Tach by examining the QRS waves, the sense circuitry can check for thepresence or the absence of respiration. The respiration rate can bedetected by monitoring the impedance across the thorax usingsubthreshold currents delivered across the active can and the highvoltage subcutaneous lead electrode and monitoring the frequency inundulation in the waveform that results from the undulations oftransthoracic impedance during the respiratory cycle. If there is noundulation, then the patent is not respiring and this lack ofrespiration can be used to confirm the QRS findings of cardiac arrest.The same technique can be used to provide information about therespiratory rate or estimate cardiac output as described in U.S. Pat.Nos. 6,095,987; 5,423,326; and 4,450,527, the entire disclosures ofwhich are incorporated herein by reference.

The canister of the present invention can be made out of titanium alloyor other presently preferred electrically active canister designs.However, it is contemplated that a malleable canister that can conformto the curvature of the patient's chest will be preferred. In this waythe patient can have a comfortable canister that conforms to the shapeof the patient's rib cage. Examples of conforming canisters are providedin U.S. Pat. No. 5,645,586, the entire disclosure of which is hereinincorporated by reference. Therefore, the canister can be made out ofnumerous materials such as medical grade plastics, metals, and alloys.In the preferred embodiment, the canister is smaller than 60 cc volumehaving a weight of less than 100 gms for long term wearability,especially in children. The canister and the lead of the S-ICD can alsouse fractal or wrinkled surfaces to increase surface area to improvedefibrillation capability. Because of the primary prevention role of thetherapy and the likely need to reach energies over 40 Joules, a featureof one embodiment is that the charge time for the therapy isintentionally left relatively long to allow capacitor charging withinthe limitations of device size. Examples of small ICD housings aredisclosed in U.S. Pat. Nos. 5,597,956 and 5,405,363, the entiredisclosures of which are herein incorporated by reference.

Different subcutaneous electrodes 13 of the present invention areillustrated in FIGS. 1-3. Turning to FIG. 1, the lead 21 for thesubcutaneous electrode is preferably composed of silicone orpolyurethane insulation. The electrode is connected to the canister atits proximal end via connection port 19 which is located on anelectrically insulated area 17 of the canister. The electrodeillustrated is a composite electrode with three different electrodesattached to the lead. In the embodiment illustrated, an optional anchorsegment 52 is attached at the most distal end of the subcutaneouselectrode for anchoring the electrode into soft tissue such that theelectrode does not dislodge after implantation.

The most distal electrode on the composite subcutaneous electrode is acoil electrode 27 that is used for delivering the high voltagecardioversion/defibrillation energy across the heart. The coilcardioversion/defibrillation electrode is about 5-10 cm in length.Proximal to the coil electrode are two sense electrodes, a first senseelectrode 25 is located proximally to the coil electrode and a secondsense electrode 23 is located proximally to the first sense electrode.The sense electrodes are spaced far enough apart to be able to have goodQRS detection. This spacing can range from 1 to 10 cm with 4 cm beingpresently preferred. The electrodes may or may not be circumferentialwith the preferred embodiment. Having the electrodes non-circumferentialand positioned outward, toward the skin surface, is a means to minimizemuscle artifact and enhance QRS signal quality. The sensing electrodesare electrically isolated from the cardioversion/defibrillationelectrode via insulating areas 29. Similar types ofcardioversion/defibrillation electrodes are currently commerciallyavailable in a transvenous configuration. For example, U.S. Pat. No.5,534,022, the entire disclosure of which is herein incorporated byreference, discloses a composite electrode with a coilcardioversion/defibrillation electrode and sense electrodes.Modifications to this arrangement are contemplated within the scope ofthe invention. One such modification is illustrated in FIG. 2 where thetwo sensing electrodes 25 and 23 are non-circumferential sensingelectrodes and one is located at the distal end, the other is locatedproximal thereto with the coil electrode located in between the twosensing electrodes. In this embodiment the sense electrodes are spacedabout 6 to about 12 cm apart depending on the length of the coilelectrode used. FIG. 3 illustrates yet a further embodiment where thetwo sensing electrodes are located at the distal end to the compositeelectrode with the coil electrode located proximally thereto. Otherpossibilities exist and are contemplated within the present invention.For example, having only one sensing electrode, either proximal ordistal to the coil cardioversion/defibrillation electrode with the coilserving as both a sensing electrode and a cardioversion/defibrillationelectrode.

It is also contemplated within the scope of the invention that thesensing of QRS waves (and transthoracic impedance) can be carried outvia sense electrodes on the canister housing or in combination with thecardioversion/defibrillation coil electrode and/or the subcutaneous leadsensing electrode(s). In this way, sensing could be performed via theone coil electrode located on the subcutaneous electrode and the activesurface on the canister housing. Another possibility would be to haveonly one sense electrode located on the subcutaneous electrode and thesensing would be performed by that one electrode and either the coilelectrode on the subcutaneous electrode or by the active surface of thecanister. The use of sensing electrodes on the canister would eliminatethe need for sensing electrodes on the subcutaneous electrode. It isalso contemplated that the subcutaneous electrode would be provided withat least one sense electrode, the canister with at least one senseelectrode, and if multiple sense electrodes are used on either thesubcutaneous electrode and/or the canister, that the best QRS wavedetection combination will be identified when the S-ICD is implanted andthis combination can be selected, activating the best sensingarrangement from all the existing sensing possibilities. Turning againto FIG. 2, two sensing electrodes 26 and 28 are located on theelectrically active surface 15 with electrical insulator rings 30 placedbetween the sense electrodes and the active surface. These canistersense electrodes could be switched off and electrically insulated duringand shortly after defibrillation/cardioversion shock delivery. Thecanister sense electrodes may also be placed on the electricallyinactive surface of the canister. In the embodiment of FIG. 2, there areactually four sensing electrodes, two on the subcutaneous lead and twoon the canister. In the preferred embodiment, the ability to changewhich electrodes are used for sensing would be a programmable feature ofthe S-ICD to adapt to changes in the patient physiology and size (in thecase of children) over time. The programming could be done via the useof physical switches on the canister, or as presently preferred, via theuse of a programming wand or via a wireless connection to program thecircuitry within the canister.

The canister could be employed as either a cathode or an anode of theS-ICD cardioversion/defibrillation system. If the canister is thecathode, then the subcutaneous coil electrode would be the anode.Likewise, if the canister is the anode, then the subcutaneous electrodewould be the cathode.

The active canister housing will provide energy and voltage intermediateto that available with ICDs and most AEDs. The typical maximum voltagenecessary for ICDs using most biphasic waveforms is approximately 750Volts with an associated maximum energy of approximately 40 Joules. Thetypical maximum voltage necessary for AEDs is approximately 2000-5000Volts with an associated maximum energy of approximately 200-360 Joulesdepending upon the model and waveform used. The S-ICD and the US-ICD ofthe present invention uses maximum voltages in the range of about 50 toabout 3500 Volts and is associated with energies of about 0.5 to about350 Joules. The capacitance of the devices can range from about 25 toabout 200 micro farads.

In another embodiment, the S-ICD and US-ICD devices provide energy witha pulse width of approximately one millisecond to approximately 40milliseconds. The devices can provide pacing current of approximatelyone milliamp to approximately 250 milliamps.

The sense circuitry contained within the canister is highly sensitiveand specific for the presence or absence of life threatening ventriculararrhythmias. Features of the detection algorithm are programmable andthe algorithm is focused on the detection of V-Fib and high rate V-Tach(>240 bpm). Although the S-ICD of the present invention may rarely beused for an actual life-threatening event, the simplicity of design andimplementation allows it to be employed in large populations of patientsat modest risk with modest cost by non-cardiac electrophysiologists.Consequently, the S-ICD of the present invention focuses mostly on thedetection and therapy of the most malignant rhythm disorders. As part ofthe detection algorithm's applicability to children, the upper raterange is programmable upward for use in children, known to have rapidsupraventricular tachycardias and more rapid ventricular fibrillation.Energy levels also are programmable downward in order to allow treatmentof neonates and infants.

Turning now to FIG. 4, the optimal subcutaneous placement of the S-ICDof the present invention is illustrated. As would be evidence to aperson skilled in the art, the actual location of the S-ICD is in asubcutaneous space that is developed during the implantation process.The heart is not exposed during this process and the heart isschematically illustrated in the figures only for help in understandingwhere the canister and coil electrode are three dimensionally located inthe left mid-clavicular line approximately at the level of theinframammary crease at approximately the 5th rib. The lead 21 of thesubcutaneous electrode traverses in a subcutaneous path around thethorax terminating with its distal electrode end at the posterioraxillary line ideally just lateral to the left scapula. This way thecanister and subcutaneous cardioversion/defibrillation electrode providea reasonably good pathway for current delivery to the majority of theventricular myocardium.

FIG. 5 illustrates a different placement of the present invention. TheS-ICD canister with the active housing is located in the left posterioraxillary line approximately lateral to the tip of the inferior portionof the scapula. This location is especially useful in children. The lead21 of the subcutaneous electrode traverses in a subcutaneous path aroundthe thorax terminating with its distal electrode end at the anteriorprecordial region, ideally in the inframammary crease. FIG. 6illustrates the embodiment of FIG. 1 subcutaneously implanted in thethorax with the proximal sense electrodes 23 and 25 located atapproximately the left axillary line with thecardioversion/defibrillation electrode just lateral to the tip of theinferior portion of the scapula.

FIG. 7 schematically illustrates the method for implanting the S-ICD ofthe present invention. An incision 31 is made in the left anterioraxillary line approximately at the level of the cardiac apex. Thisincision location is distinct from that chosen for S-ICD placement andis selected specifically to allow both canister location more mediallyin the left inframammary crease and lead positioning more posteriorlyvia the introducer set (described below) around to the left posterioraxillary line lateral to the left scapula. That said, the incision canbe anywhere on the thorax deemed reasonable by the implanting physicianalthough in the preferred embodiment, the S-ICD of the present inventionwill be applied in this region. A subcutaneous pathway 33 is thencreated medially to the inframammary crease for the canister andposteriorly to the left posterior axillary line lateral to the leftscapula for the lead.

The S-ICD canister 11 is then placed subcutaneously at the location ofthe incision or medially at the subcutaneous region at the leftinframammary crease. The subcutaneous electrode 13 is placed with aspecially designed curved introducer set 40 (see FIG. 8). The introducerset comprises a curved trocar 42 and a stiff curved peel away sheath 44.The peel away sheath is curved to allow for placement around the ribcage of the patient in the subcutaneous space created by the trocar. Thesheath has to be stiff enough to allow for the placement of theelectrodes without the sheath collapsing or bending. Preferably thesheath is made out of a biocompatible plastic material and is perforatedalong its axial length to allow for it to split apart into two sections.The trocar has a proximal handle 41 and a curved shaft 43. The distalend 45 of the trocar is tapered to allow for dissection of asubcutaneous path 33 in the patient. Preferably, the trocar iscannulated having a central Lumen 46 and terminating in an opening 48 atthe distal end. Local anesthetic such as lidocaine can be delivered, ifnecessary, through the lumen or through a curved and elongated needledesigned to anesthetize the path to be used for trocar insertion shouldgeneral anesthesia not be employed. The curved peel away sheath 44 has aproximal pull tab 49 for breaking the sheath into two halves along itsaxial shaft 47. The sheath is placed over a guidewire inserted throughthe trocar after the subcutaneous path has been created. Thesubcutaneous pathway is then developed until it terminatessubcutaneously at a location that, if a straight line were drawn fromthe canister location to the path termination point the line wouldintersect a substantial portion of the left ventricular mass of thepatient. The guidewire is then removed leaving the peel away sheath. Thesubcutaneous lead system is then inserted through the sheath until it isin the proper location. Once the subcutaneous lead system is in theproper location, the sheath is split in half using the pull tab 49 andremoved. If more than one subcutaneous electrode is being used, a newcurved peel away sheath can be used for each subcutaneous electrode.

The S-ICD will have prophylactic use in adults where chronictransvenous/epicardial ICD lead systems pose excessive risk or havealready resulted in difficulty, such as sepsis or lead fractures. It isalso contemplated that a major use of the S-ICD system of the presentinvention will be for prophylactic use in children who are at risk forhaving fatal arrhythmias, where chronic transvenous lead systems posesignificant management problems. Additionally, with the use of standardtransvenous ICDs in children, problems develop during patient growth inthat the lead system does not accommodate the growth. FIG. 9 illustratesthe placement of the S-ICD subcutaneous lead system such that theproblem that growth presents to the lead system is overcome. The distalend of the subcutaneous electrode is placed in the same location asdescribed above providing a good location for the coilcardioversion/defibrillation electrode 27 and the sensing electrodes 23and 25. The insulated lead 21, however, is no longer placed in a tautconfiguration. Instead, the lead is serpiginously placed with aspecially designed introducer trocar and sheath such that it hasnumerous waves or bends. As the child grows, the waves or bends willstraighten out lengthening the lead system while maintaining properelectrode placement. Although it is expected that fibrous scarringespecially around the defibrillation coil will help anchor it intoposition to maintain its posterior position during growth, a lead systemwith a distal tine or screw electrode anchoring system 52 can also beincorporated into the distal tip of the lead to facilitate leadstability (see FIG. 1). Other anchoring systems can also be used such ashooks, sutures, or the like.

FIGS. 10 and 11 illustrate another embodiment of the present S-ICDinvention. In this embodiment there are two subcutaneous electrodes 13and 13′ of opposite polarity to the canister. The additionalsubcutaneous electrode 13′ is essentially identical to the previouslydescribed electrode. In this embodiment the cardioversion/defibrillationenergy is delivered between the active surface of the canister and thetwo coil electrodes 27 and 27′. Additionally, provided in the canisteris means for selecting the optimum sensing arrangement between the foursense electrodes 23, 23′, 25, and 25′. The two electrodes aresubcutaneously placed on the same side of the heart. As illustrated inFIG. 6, one subcutaneous electrode 13 is placed inferiorly and the otherelectrode 13′ is placed superiorly. It is also contemplated with thisdual subcutaneous electrode system that the canister and onesubcutaneous electrode are the same polarity and the other subcutaneouselectrode is the opposite polarity.

Turning now to FIGS. 12 and 13, further embodiments are illustratedwhere the canister 11 of the S-ICD of the present invention is shaped tobe particularly useful in placing subcutaneously adjacent and parallelto a rib of a patient. The canister is long, thin, and curved to conformto the shape of the patient's rib. In the embodiment illustrated in FIG.12, the canister has a diameter ranging from about 0.5 cm to about 2 cmwithout 1 cm being presently preferred. Alternatively, instead of havinga circular cross sectional area, the canister could have a rectangularor square cross sectional area as illustrated in FIG. 13 without fallingoutside of the scope of the present invention. The length of thecanister can vary depending on the size of the patient's thorax. In anembodiment, the canister is about 5 cm to about 40 cm long. The canisteris curved to conform to the curvature of the ribs of the thorax. Theradius of the curvature will vary depending on the size of the patient,with smaller radiuses for smaller patients and larger radiuses forlarger patients. The radius of the curvature can range from about 5 cmto about 35 cm depending on the size of the patient. Additionally, theradius of the curvature need not be uniform throughout the canister suchthat it can be shaped closer to the shape of the ribs. The canister hasan active surface, 15 that is located on the interior (concave) portionof the curvature and an inactive surface 16 that is located on theexterior (convex) portion of the curvature. The leads of theseembodiments, which are not illustrated except for the attachment port 19and the proximal end of the lead 21, can be any of the leads previouslydescribed above, with the lead illustrated in FIG. 1 being presentlypreferred.

The circuitry of this canister is similar to the circuitry describedabove. Additionally, the canister can optionally have at least one senseelectrode located on either the active surface of the inactive surfaceand the circuitry within the canister can be programmable as describedabove to allow for the selection of the best sense electrodes. It ispresently preferred that the canister have two sense electrodes 26 and28 located on the inactive surface of the canisters as illustrated,where the electrodes are spaced from about 1 to about 10 cm apart with aspacing of about 3 cm being presently preferred. However, the senseelectrodes can be located on the active surface as described above.

It is envisioned that the embodiment of FIG. 12 will be subcutaneouslyimplanted adjacent and parallel to the left anterior 5th rib, eitherbetween the 4th and 5th ribs or between the 5th and 6th ribs. Howeverother locations can be used.

Another component of the S-ICD of the present invention is a cutaneoustest electrode system designed to simulate the subcutaneous high voltageshock electrode system as well as the QRS cardiac rhythm detectionsystem. This test electrode system is comprised of a cutaneous patchelectrode of similar surface area and impedance to that of the S-ICDcanister itself together with a cutaneous strip electrode comprising adefibrillation strip as well as two button electrodes for sensing of theQRS. Several cutaneous strip electrodes are available to allow fortesting various bipole spacings to optimize signal detection comparableto the implantable system.

FIGS. 14 to 18 depict particular US-ICD embodiments of the presentinvention. The various sensing, shocking and pacing circuitry, describedin detail above with respect to the S-ICD embodiments, may additionallybe incorporated into the following US-ICD embodiments. Furthermore,particular aspects of any individual S-ICD embodiment discussed abovemay be incorporated, in whole or in part, into the US-ICD embodimentsdepicted in the following figures.

Turning now to FIG. 14, the US-ICD of the present invention isillustrated. The US-ICD consists of a curved housing 1211 with a firstand second end. The first end 1413 is thicker than the second end 1215.This thicker area houses a battery supply, capacitor and operationalcircuitry for the US-ICD. The circuitry will be able to monitor cardiacrhythms for tachycardia and fibrillation, and if detected, will initiatecharging the capacitor and then delivering cardioversion/defibrillationenergy through the two cardioversion/defibrillating electrodes 1417 and1219 located on the outer surface of the two ends of the housing. Thecircuitry can provide cardioversion/defibrillation energy in differenttypes of waveforms. In one embodiment, a 100 uF biphasic waveform isused of approximately 10-20 ms total duration and with the initial phasecontaining approximately ⅔ of the energy, however, any type of waveformcan be utilized such as monophasic, biphasic, multiphasic or alternativewaveforms as is known in the art.

The housing of the present invention can be made out of titanium alloyor other presently preferred ICD designs. It is contemplated that thehousing is also made out of biocompatible plastic materials thatelectronically insulate the electrodes from each other. However, it iscontemplated that a malleable canister that can conform to the curvatureof the patient's chest will be preferred. In this way the patient canhave a comfortable canister that conforms to the unique shape of thepatient's rib cage. Examples of conforming ICD housings are provided inU.S. Pat. No. 5,645,586, the entire disclosure of which is hereinincorporated by reference. In the preferred embodiment, the housing iscurved in the shape of a 5^(th) rib of a person. Because there are manydifferent sizes of people, the housing will come in differentincremental sizes to allow a good match between the size of the rib cageand the size of the US-ICD. The length of the US-ICD will range fromabout 15 to about 50 cm. Because of the primary preventative role of thetherapy and the need to reach energies over 40 Joules, a feature of thepreferred embodiment is that the charge time for the therapy,intentionally be relatively long to allow capacitor charging within thelimitations of device size.

The thick end of the housing is currently needed to allow for theplacement of the battery supply, operational circuitry, and capacitors.It is contemplated that the thick end will be about 0.5 cm to about 2 cmwide with about 1 cm being presently preferred. As microtechnologyadvances, the thickness of the housing will become smaller.

The two cardioversion/defibrillation electrodes on the housing are usedfor delivering the high voltage cardioversion/defibrillation energyacross the heart. In the preferred embodiment, thecardioversion/defibrillation electrodes are coil electrodes, however,other cardioversion/defibrillation electrodes could be used such ashaving electrically isolated active surfaces or platinum alloyelectrodes. The coil cardioversion/defibrillation electrodes are about5-10 cm in length. Located on the housing between the twocardioversion/defibrillation electrodes are two sense electrodes 1425and 1427. The sense electrodes are spaced far enough apart to be able tohave good QRS detection. This spacing can range from 1 to 10 cm with 4cm being presently preferred. The electrodes may or may not becircumferential with the preferred embodiment. Having the electrodesnon-circumferential and positioned outward, toward the skin surface, isa means to minimize muscle artifact and enhance QRS signal quality. Thesensing electrodes are electrically isolated from thecardioversion/defibrillation electrode via insulating areas 1423.Analogous types of cardioversion/defibrillation electrodes are currentlycommercially available in a transvenous configuration. For example, U.S.Pat. No. 5,534,022, the entire disclosure of which is hereinincorporated by reference, discloses a composite electrode with a coilcardioversion/defibrillation electrode and sense electrodes.Modifications to this arrangement are contemplated within the scope ofthe invention. One such modification is to have the sense electrodes atthe two ends of the housing and have the cardioversion/defibrillationelectrodes located in between the sense electrodes. Another modificationis to have three or more sense electrodes spaced throughout the housingand allow for the selection of the two best sensing electrodes. If threeor more sensing electrodes are used, then the ability to change whichelectrodes are used for sensing would be a programmable feature of theUS-ICD to adapt to changes in the patient physiology and size over time.The programming could be done via the use of physical switches on thecanister, or as presently preferred, via the use of a programming wandor via a wireless connection to program the circuitry within thecanister.

Turning now to FIG. 15, the optimal subcutaneous placement of the US-ICDof the present invention is illustrated. As would be evident to a personskilled in the art, the actual location of the US-ICD is in asubcutaneous space that is developed during the implantation process.The heart is not exposed during this process and the heart isschematically illustrated in the figures only for help in understandingwhere the device and its various electrodes are three dimensionallylocated in the thorax of the patient. The US-ICD is located between theleft mid-clavicular line approximately at the level of the inframammarycrease at approximately the 5^(th) rib and the posterior axillary line,ideally just lateral to the left scapula. This way the US-ICD provides areasonably good pathway for current delivery to the majority of theventricular myocardium.

FIG. 16 schematically illustrates the method for implanting the US-ICDof the present invention. An incision 1631 is made in the left anterioraxillary line approximately at the level of the cardiac apex. Asubcutaneous pathway is then created that extends posteriorly to allowplacement of the US-ICD. The incision can be anywhere on the thoraxdeemed reasonable by the implanting physician although in the preferredembodiment, the US-ICD of the present invention will be applied in thisregion. The subcutaneous pathway is created medially to the inframammarycrease and extends posteriorly to the left posterior axillary line. Thepathway is developed with a specially designed curved introducer 1742(see FIG. 17). The trocar has a proximal handle 1641 and a curved shaft1643. The distal end 1745 of the trocar is tapered to allow fordissection of a subcutaneous path in the patient. Preferably, the trocaris cannulated having a central lumen 1746 and terminating in an opening1748 at the distal end. Local anesthetic such as lidocaine can bedelivered, if necessary, through the lumen or through a curved andelongated needle designed to anesthetize the path to be used for trocarinsertion should general anesthesia not be employed. Once thesubcutaneous pathway is developed, the US-ICD is implanted in thesubcutaneous space, the skin incision is closed using standardtechniques.

As described previously, the US-ICDs of the present invention vary inlength and curvature. The US-ICDs are provided in incremental sizes forsubcutaneous implantation in different sized patients. Turning now toFIG. 18, a different embodiment is schematically illustrated in explodedview which provides different sized US-ICDs that are easier tomanufacture. The different sized US-ICDs will all have the same sizedand shaped thick end 1413. The thick end is hollow inside allowing forthe insertion of a core operational member 1853. The core membercomprises a housing 1857 which contains the battery supply, capacitorand operational circuitry for the US-ICD. The proximal end of the coremember has a plurality of electronic plug connectors. Plug connectors1861 and 1863 are electronically connected to the sense electrodes viapressure fit connectors (not illustrated) inside the thick end which arestandard in the art. Plug connectors 1865 and 1867 are alsoelectronically connected to the cardioverter/defibrillator electrodesvia pressure fit connectors inside the thick end. The distal end of thecore member comprises an end cap 1855, and a ribbed fitting 1859 whichcreates a water-tight seal when the core member is inserted into opening1851 of the thick end of the US-ICD.

The S-ICD and US-ICD, in alternative embodiments, have the ability todetect and treat atrial rhythm disorders, including atrial fibrillation.The S-ICD and US-ICD have two or more electrodes that provide afar-field view of cardiac electrical activity that includes the abilityto record the P-wave of the electrocardiogram as well as the QRS. Onecan detect the onset and offset of atrial fibrillation by referencing tothe P-wave recorded during normal sinus rhythm and monitoring for itschange in rate, morphology, amplitude and frequency content. Forexample, a well-defined P-wave that abruptly disappeared and wasreplaced by a low-amplitude, variable morphology signal would be astrong indication of the absence of sinus rhythm and the onset of atrialfibrillation. In an alternative embodiment of a detection algorithm, theventricular detection rate could be monitored for stability of the R-Rcoupling interval. In the examination of the R-R interval sequence,atrial fibrillation can be recognized by providing a near constantirregularly irregular coupling interval on a beat-by-beat basis. An R-Rinterval plot during AF appears “cloudlike” in appearance when severalhundred or thousands of R-R intervals are plotted over time whencompared to sinus rhythm or other supraventricular arrhythmias.Moreover, a distinguishing feature compared to other rhythms that areirregularly irregular, is that the QRS morphology is similar on abeat-by-beat basis despite the irregularity in the R-R couplinginterval. This is a distinguishing feature of atrial fibrillationcompared to ventricular fibrillation where the QRS morphology varies ona beat-by-beat basis. In yet another embodiment, atrial fibrillation maybe detected by seeking to compare the timing and amplitude relationshipof the detected P-wave of the electrocardiogram to the detected QRS(R-wave) of the electrocardiogram. Normal sinus rhythm has a fixedrelationship that can be placed into a template matching algorithm thatcan be used as a reference point should the relationship change.

In other aspects of the atrial fibrillation detection process, one mayinclude alternative electrodes that may be brought to bear in the S-ICDor US-ICD systems either by placing them in the detection algorithmcircuitry through a programming maneuver or by manually adding suchadditional electrode systems to the S-ICD or US-ICD at the time ofimplant or at the time of follow-up evaluation. One may also useelectrodes for the detection of atrial fibrillation that may or may notalso be used for the detection of ventricular arrhythmias given thedifferent anatomic locations of the atria and ventricles with respect tothe S-ICD or US-ICD housing and surgical implant sites.

Once atrial fibrillation is detected, the arrhythmia can be treated bydelivery of a synchronized shock using energy levels up to the maximumoutput of the device therapy for terminating atrial fibrillation or forother supraventricular arrhythmias. The S-ICD or US-ICD electrode systemcan be used to treat both atrial and ventricular arrhythmias not onlywith shock therapy but also with pacing therapy. In a further embodimentof the treatment of atrial fibrillation or other atrial arrhythmias, onemay be able to use different electrode systems than what is used totreat ventricular arrhythmias. Another embodiment would be to allow fordifferent types of therapies (amplitude, waveform, capacitance, etc.)for atrial arrhythmias compared to ventricular arrhythmias.

The core member of the different sized and shaped US-ICD will all be thesame size and shape. That way, during an implantation procedure,multiple sized US-ICDs can be available for implantation, each onewithout a core member. Once the implantation procedure is beingperformed, then the correct sized US-ICD can be selected and the coremember can be inserted into the US-ICD and then programmed as describedabove. Another advantage of this configuration is when the batterywithin the core member needs replacing it can be done without removingthe entire US-ICD.

Post-shock bradycardia is a common after-effect of shocking the heartfor cardioversion/defibrillation therapy. Symptoms related to low bloodpressure may result from post-shock bradycardia whenever the heart ratefalls below approximately 30 to approximately 50 beats per minute.Accordingly, it is often desirable to provide anti-bradycardia pacing tocorrect the symptoms resulting from bradycardia.

To ensure adequate pacing capture of the heart through a subcutaneousonly lead system, pacing therapy can be considerably enhanced (i.e.,require less energy and voltage) by using either a monophasic or abiphasic waveform for pacing.

FIG. 19 is a graph that shows an embodiment of the example of a biphasicwaveform for use in anti-bradycardia pacing applications in subcutaneousimplantable cardioverter-defibrillators (“S-ICD”) in an embodiment ofthe present invention. As shown in FIG. 19, the biphasic waveform isplotted as a function of current versus time.

In an embodiment, the biphasic waveform 1902 comprises a positiveportion 1904, a negative portion 1906 and a transition portion 1908. Inan embodiment, both the positive portion 1904 and the negative portion1906 are substantially rectangular in shape. The positive portion 1904of the biphasic waveform 1902 comprises an initial positive current1910, a positive fixed current 1912 and a final positive current 1914.The negative portion 1906 of the biphasic waveform 1902 comprises aninitial negative current 1916, a negative fixed current 1918 and a finalnegative current 1920. In an embodiment, the polarities of the biphasicwaveform 1902 can be reversed such that the negative portion 1906precedes the positive portion 1904 in time.

As shown in FIG. 19, the biphasic waveform 1902 is initially at zerocurrent. Upon commencement of the anti-bradycardia pacing, a current ofpositive polarity is provided and the biphasic waveform 1902 rises tothe initial positive current 1910. Next, the current of the biphasicwaveform 1902 remains at a constant level along the positive fixedcurrent 1912. The positive portion 1904 of the biphasic waveform 1902 isthen truncated and a negative current is provided. The biphasic waveform1902 then undergoes a relatively short transition portion 1908 where thecurrent is approximately zero. Next, the biphasic waveform 1902 isincreased (in absolute value) in the opposite (negative) polarity to theinitial negative current 1916. After reaching its maximum negativecurrent (in absolute value), the current of the biphasic waveform 1902remains at a constant level along the negative fixed current 1918. Afterthe negative portion 1906 of the biphasic waveform 1902 is truncated atthe final negative current 1914, the biphasic waveform 1902 returns tozero.

The total amount of time that the biphasic waveform 1902 comprises isknown as the “pulse width.” In an embodiment, the pulse width of thebiphasic waveform can range from approximately 1 millisecond toapproximately 40 milliseconds. The total amount of energy delivered is afunction of the pulse width and the absolute value of the current.

An example of one embodiment of the biphasic waveform 1902 will now bedescribed. In this embodiment, the amplitude of the initial positivecurrent 1910 can range from approximately one to approximately 250milliamps. Similarly, the amplitude of the initial negative current 1916can range from approximately one to approximately 250 milliamps.

In the example, the pulse width of the biphasic waveform 1902 can rangefrom approximately 1 millisecond to approximately 40 milliseconds. Inaddition, the implantable cardioverter-defibrillator employs biphasicanti-bradycardia pacing at rates of approximately 20 to approximately120 stimuli/minute for severe bradycardia episodes although programmingof higher pacing rates up to 120 stimuli/minute is also possible.

FIG. 20 is a graph that shows an embodiment of the example of amonophasic waveform for use in anti-bradycardia pacing applications insubcutaneous implantable cardioverter-defibrillators (“S-ICD”) in anembodiment of the present invention. As shown in FIG. 20, the monophasicwaveform is plotted as a function of current versus time.

In an embodiment, the monophasic waveform 2002 comprises an initialpositive current 2004, a positive fixed current 2006 and a finalpositive current 2008. In an embodiment, the monophasic waveform 2002 issubstantially rectangular in shape. In an embodiment, the polarities ofthe monophasic waveform 2002 can be reversed such that the waveform 2002is negative in polarity.

As shown in FIG. 20, the monophasic waveform 2002 is initially at zerocurrent. Upon commencement of the anti-bradycardia pacing, a current ofpositive polarity is provided and the monophasic waveform 2002 rises tothe initial positive current 2004. Next, the current of the monophasicwaveform 2002 remains at a constant level along the positive fixedcurrent 1906. The monophasic waveform 2002 is then truncated.

The total amount of time that the monophasic waveform 2002 comprises isknown as the “pulse width.” In an embodiment, the pulse width of themonophasic waveform can range from approximately 1 millisecond toapproximately 40 milliseconds. The total amount of energy delivered is afunction of the pulse width and the absolute value of the current.

An example of one embodiment of the monophasic waveform 2002 will now bedescribed. In this embodiment, the amplitude of the initial positivecurrent 2004 can range from approximately one to approximately 250milliamps.

In the example, the pulse width of the monophasic waveform 2002 canrange from approximately 1 millisecond to approximately 40 milliseconds.In addition, the implantable cardioverter-defibrillator employsmonophasic anti-bradycardia pacing at rates of approximately 20 toapproximately 120 stimuli/minute for severe bradycardia episodesalthough programming of higher pacing rates up to 120 stimuli/minute isalso possible. In order to maintain these rates, in one embodiment ofthe invention, the power supply continues to operate to maintain asufficient voltage to deliver a constant current.

Although it possible for the present invention to provide standard VVIpacing at predetermined or preprogrammed rates, one embodiment providesanti-bradycardia pacing only for bradycardia or post-shock bradycardia.To avoid frequent anti-bradycardia pacing at 50 stimuli/minute but toprovide this rate in case of emergencies, a hysteresis detection triggercan be employed at lower rates, typically in the range of approximately20 to approximately 40 stimuli/minute. For example, a default settingmay be set at approximately 20 stimuli/minute (i.e., the equivalent of a3 second pause), and the invention providing VVI pacing at a rate ofapproximately 50 stimuli/minute only when such a pause occurs. Inanother embodiment, the invention can provide physiologic pacing in aVVIR mode of operation in response to a certain activity, respiration,pressure or oxygenation sensor.

The S-ICD and US-ICD devices and methods of the present invention may beembodied in other specific forms without departing from the teachings oressential characteristics of the invention. The described embodimentsare therefore to be considered in all respects as illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than by the foregoing description, and all changes whichcome within the meaning and range of equivalency of the claims aretherefore to be embraced therein.

1. A method of treating the heart of a patient comprising applying afirst constant current defibrillation stimulus across a portion ofpatient tissue between first and second electrodes both disposedsubcutaneously, nonvascularly, and exclusive of the heart; wherein thefirst electrode is disposed on a canister containing circuitry forgenerating the first constant current, and the second electrode isdisposed on a lead electrode assembly electrically coupled to thecircuitry in the canister.
 2. The method of claim 1, further comprisingdetermining whether the patient's cardiac rhythm is normal, wherein thestep of applying a first constant current is performed if the patient'scardiac rhythm is abnormal.
 3. The method of claim 1, wherein the firstconstant current is provided at a time to convert the patient intonormal cardiac rhythm.
 4. The method of claim 1, further comprisingapplying a second constant current across the portion of tissue usingthe first and second electrodes, wherein the second constant current isof opposite sign from the first constant current.
 5. The method of claim1, wherein a line drawn from the first electrode to the second electrodeintersects a portion of the patient's heart.
 6. A method of treating theheart of a patient comprising applying a first constant currentdefibrillation stimulus across a portion of patient tissue between firstand second electrodes both disposed subcutaneously, nonvascularly, andexclusive of the heart, wherein the first electrode is disposed alongthe left axillary line of the patient, and the second electrode isdisposed medial from the first electrode.
 7. The method of claim 6,wherein the first electrode is disposed along the inframammary crease ofthe patient.
 8. The method of claim 7, wherein a line drawn from thefirst electrode to the second electrode intersects a portion of thepatient's heart.
 9. A method of treating the heart of a patientcomprising applying a first constant current across a portion of patienttissue between first and second electrodes both disposed subcutaneously,nonvascularly, and exclusive of the heart, wherein the first electrodeis disposed along the inframammary crease of the patient.
 10. The methodof claim 9, wherein the first electrode is disposed on a canistercontaining circuitry for generating the first constant current, and thesecond electrode is disposed on a lead electrode assembly electricallycoupled to the circuitry in the canister.
 11. The method of claim 9,wherein a line drawn from the first electrode to the second electrodeintersects a portion of the patient's heart.
 12. A method of treatingthe heart of a patient comprising providing a biphasic constant currentdefibrillation stimulus across a portion of patient tissue includingheart tissue between first and second electrodes implanted in the torsoof the patient such that the first and second electrodes do not contactthe patient's heart or reside in the patient's vasculature; wherein thefirst electrode is disposed on a canister containing circuitry forgenerating the biphasic constant current defibrillation stimulus, andthe second electrode is disposed on a lead electrode assemblyelectrically coupled to the circuitry in the canister, and the leadassembly does not reside in any intrathoracic blood vessel of thepatient.
 13. The method of claim 12, further comprising determiningwhether the patient's cardiac rhythm is normal, wherein the step ofapplying a biphasic constant current defibrillation stimulus isperformed if the patient's cardiac rhythm is abnormal.
 14. The method ofclaim 12, wherein biphasic constant current defibrillation stimulus isprovided at a time to convert the patient into normal cardiac rhythm.15. The method of claim 12, wherein a line drawn from the firstelectrode to the second electrode intersects a portion of the patient'sheart.
 16. A method of treating the heart of a patient comprisingproviding a biphasic constant current defibrillation stimulus across aportion of patient tissue including heart tissue between first andsecond electrodes implanted in the torso of the patient such that thefirst and second electrodes do not contact the patient's heart or residein the patient's vasculature, wherein the first electrode is disposedalong the left axillary line of the patient, and the second electrode isdisposed medial from the first electrode.
 17. The method of claim 16,wherein the first electrode is disposed along the inframammary crease ofthe patient.
 18. The method of claim 17, wherein a line drawn from thefirst electrode to the second electrode intersects a portion of thepatient's heart.
 19. A method of treating the heart of a patientcomprising providing a biphasic constant current signal across a portionof patient tissue including heart tissue between first and secondelectrodes implanted in the torso of the patient such that the first andsecond electrodes do not contact the patient's heart or reside in thepatient's vasculature, wherein the first electrode is disposed along theinframammary crease of the patient.
 20. The method of claim 19, whereina line drawn from the first electrode to the second electrode intersectsa portion of the patient's heart.
 21. A method of treating the heart ofa patient comprising providing a biphasic constant current signal acrossa portion of patient tissue including heart tissue between first andsecond electrodes implanted in the torso of the patient such that thefirst and second electrodes do not contact the patient's heart or residein the patient's vasculature, the first and second electrodes being partof an implantable medical system comprising a canister and a single leadassembly which does not reside in any intrathoracic blood vessel of thepatient; wherein the biphasic constant current signal includes a firstportion having a first duration and a first magnitude and a secondportion having a second duration and a second magnitude; and wherein thefirst portion has a magnitude in the range of about one to two-hundredfifty milliamps, and the second portion has a magnitude in the range ofabout one to two-hundred fifty milliamps and the biphasic constantcurrent signal, from the start of the first portion to the end of thesecond portion, has a duration in the range of about one to fortymilliseconds.
 22. The method of claim 21, wherein the first electrode isdisposed on a canister containing circuitry for generating the biphasicconstant current signal, and the second electrode is disposed on a leadelectrode assembly electrically coupled to the circuitry in thecanister.
 23. A method of treating the heart of a patient comprisingproviding a constant current signal across a portion of patient tissueincluding heart tissue between first and second electrodes implanted inthe torso of the patient such that the first and second electrodes donot contact the patient's heart or reside in the patient's vasculature;wherein: the first electrode is disposed on a canister containingcircuitry for generating the constant current signal, and the secondelectrode is disposed on a lead electrode assembly electrically coupledto the circuitry in the canister; the first electrode is disposed alongthe left axillary line of the patient and the second electrode isdisposed medial from the first electrode; the first electrode isdisposed along the inframammary crease of the patient; and a line drawnfrom the first electrode to the second electrode intersects a portion ofthe patient's heart.
 24. The method of claim 23, wherein the constantcurrent signal is provided at a time to convert the patient into normalcardiac rhythm.
 25. The method of claim 23, further comprisingdetermining whether the patient's cardiac rhythm is normal, wherein thestep of applying a constant current signal is performed if the patient'scardiac rhythm is abnormal.